This invention relates to surface acoustic wave devices which utilize ferroelectric crystals.
A surface acoustic wave is one of several types of wave motion in which acoustic energy can travel through a solid medium. Whereas bulk acoustic waves propagate through the interior of an acoustically conductive medium as either longitudinal or shear waves, surface waves are a complex mixture of shear and longitudinal wave motions which may occur only in the presence of a stress free boundary condition, such as at a surface. Because of this boundary condition, surface wave energy extends only a few wavelengths into the bulk of the propagating medium. This surface concentration of wave energy makes possible a variety of applications for surface wave devices in the field of electronics, such as signal filtering, the amplification of weak signals, the storage of signals in delay lines, the provision of highly accurate frequency references, and the detection of changes in a physical parameter, such as pressure or temperature.
Many practical applications for surface acoustic wave devices have appeared since the development of the interdigital transducer, which can effeciently convert an electrical signal into a surface acoustic wave and vice versa. The simplest interdigital transducer consists of a pair of interleaved electrodes which are placed in electrical contact with the surface of a piezoelectric material. When such a material is physically distorted, it produces an internal electric field. Conversely, if an electric field is applied to a piezoelectric material, the material will expand or contract, depending upon the polarity of the applied field. Because of this phenomenon, when a rapidly changing electrical signal is applied to a piezoelectric material through an interdigital transducer, the material will vibrate in response, thereby generating a surface acoustic wave. A single pair of interdigital electrodes will not produce surface acoustic waves with great efficiency, but multiple pairs of electrodes, when placed in an interdigitating pattern, will each excite an acoustic wave and, if the spacing between the electrodes is properly related to the desired acoustic wavelength, the separately excited waves can be made to reinforce one another and produce a suitably large acoustic signal.
If the frequency of the electrical input signal is altered from the ideal value, the individual excitations from each pair of electrodes will tend to cancel. The larger the number of electrode pairs, the more a slight change in frequency will tend to cancel the excited wave. This characteristic can be exploited in a filter by providing an input interdigital transducer and an output interdigital transducer in a delay line configuration, to sort out electrical signals of one frequency from other signals. By tapering the spacing between the electrodes and the length of the electrodes, the coupling of any particular pair of electrodes can be varied and coupling as a function of frequency can be controlled.
Considerable interest has arisen in the use of surface acoustic waves for signal processing functions, an application which requires interdigital transducers having electrode geometries and spacings designed to sample the acoustic signal at particular points. In the case of an analog signal whose frequency varies as some particular function of time, for example, an interdigital transducer can be tailored to recognize the signal by selecting proper spacing and electrode lengths at different positions along the transducer.
Another signal processing application for surface acoustic waves involves the tapped delay line, which responds to particular digital codes. A tapped delay line includes a uniform array of electrode pairs deposited on a suitable substrate. A signal in the form of a short pulse introduced by means of a wideband transducer at one end of the delay line will generate an electrical output at each electrode pair in the array as it propagates past that pair. If the electrical outputs of these electrodes are all connected together, a series of successive pulses, analogous to ones in binary code, will result. If some of the connections are reversed, however, the corresponding pulses will be reversed in polarity and will be equivalent to zeros in the binary system. This technique can be utilized to produce a coded digital signal for transmission and to recognize such a received signal, since an identically connected array of transducers will produce a large electrical output signal when the coded acoustic signal is sent into the array. Such a device can be made programmable by providing a switch for each electrode pair to control the polarity of that pair. This addition makes feasible the production of high speed signal processors with a large number of taps which can be rapidly switched to respond to complex and changing coded signals.
The development of these surface acoustic wave devices has been limited by the techniques available in the art for fabricating interdigital transducers. In the case of an analog filter, for example, the electrode sizes and spacing must be fixed at the time of fabricating the transducer and cannot readily be later altered to adapt the filter to respond to different signals than the particular one for which the filter was designed. One of the difficulties which has arisen with programmable tapped delay lines involves the switches used for changing the polarity of the individual electrode pairs. In prior art designs, these switches must handle radio frequency signals, making the design of such switches more complicated and reducing the efficiency of the device. These lamitations of the prior art are illustrative of some of the problems which are solved by the techniques of the present invention.